The present invention relates to a heat exchanger, and more particularly to a heat exchanger for using a hot liquid to warm a substance that is cold, fluid, and foamy, and that contains solid particles and gas.
The invention has been designed in the context of a particular application, specifically that of treating animal excreta, and more particularly pig manure, in an installation for dehydrating liquid manure.
By way of example, an installation of this type is described in international patent application WO 93/16005 in the name of the Applicant, to which reference can be made where necessary.
In the installation constituting the subject matter of that patent application, the substance for treatment is spread out continuously in the form of a thin layer on the top face of a heat exchanger wall which is heated to a temperature that is high enough to cause the volatile components contained in the substance, and in particular the water, to evaporate quickly; the solid and dry residue is removed as it forms on said face by scraping; the heat exchanger surface is heated by means of steam that results from evaporation, the steam being compressed mechanically and then put into contact with the bottom face of the wall so as to condense thereon, the distillate subsequently being removed.
That extraction method is particularly low in cost because the energy released by the condensation is used on the other side of the heat exchanger wall for evaporation.
In order to further improve the thermal efficiency of the method, it is advantageous to preheat the substance that is to be treated, specifically liquid manure, prior to bringing it to the evaporator, and to do so using the hot distillate that comes from the evaporator.
It is in that context that the present invention has been developed, the heat exchanger being intended to preheat the liquid manure that is to be dehydrated by evaporation, the hot liquid used for preheating being the distillate produced by the dehydration treatment.
The design of such a heat exchanger has encountered technical difficulties which are associated with the very peculiar nature of liquid manure.
Liquid manure is a fluid, semiliquid substance having the consistency of slurry containing solid particles and gas, and it is extremely foamy or frothy. It comprises a medium which is entirely heterogeneous, containing heavy materials which settle quickly and light materials which tend to float; this substance is the seat of very significant outgassing during warming, with the volume of gas producedxe2x80x94essentially carbon dioxide (CO2)xe2x80x94being two to ten times greater than the volume of the manure; foam is also formed in large quantities, in a medium which is highly aggressive.
Tests were initially made using conventional type heat exchangers and they did not give satisfaction, with faulty operation being observed very quickly, due in particular to ducts becoming clogged.
The present invention seeks to resolve those difficulties by proposing a heat exchanger of the above-mentioned type which is adapted to warming a foamy semiliquid substance containing solid particles and gas, in particular such as liquid manure, the heat exchanger nevertheless being simple in design technically speaking and of moderate cost price, easy to maintain, and capable of operating under satisfactory conditions concerning reliability, throughput, and efficiency.
These various objectives are achieved by the fact that the heat exchanger of the present invention comprises a plurality of unit heat exchangers connected in series, each constituted by a horizontal tube having a heat conducting wall, in which the substance is caused to circulate from one end to the other, and a concentric tubular case surrounding said tube in which the hot liquid is caused to circulate from one end to the other in the opposite direction to the circulation of said substance, and that inside said tube there is coaxially mounted a rotary shaft carrying a helical brush forming an Archimedes"" screw thread that rubs against the inside wall of the tube to ensure that the substance is transported therealong.
Furthermore, according to various additional but non-limiting characteristics of the invention:
said unit heat exchangers are disposed substantially in a common vertical plane one above another, with the substance circulating downwards, i.e. from the top unit heat exchanger to the bottom unit heat exchanger of the series, while the hot liquid circulates in the opposite direction, i.e. upwards;
the downstream end of said tube has a gas removal fitting;
said rotary shaft is mounted inside the tube via a floating mount, the shaft being centered automatically within the tube by the helical brush bearing against the inside wall of the tube;
the shaft is rotated by means of a coupling member which leaves the shaft with a certain amount of freedom to move radially;
a helical gasket is interposed between the tube and its case so as to channel the hot liquid and cause it to circulate in the form of a helical sheet;
said gasket is an inflatable tube of synthetic rubber;
all of the helical brushes have screw pitches with the same handedness, while pairs of adjacent shafts are rotated in opposite directions; and
each of the shafts carries a driving sprocket, all of the shafts being driven by a motor via a chain engaging the set of sprockets.
In one possible application of the invention, the heat exchanger is for preheating liquid manure that is to be dehydrated by evaporation, the hot liquid being the distillate produced by the dehydration treatment.
Other characteristics and advantages of the invention appear from the following description and the accompanying drawings which show a preferred embodiment merely by way of non-limiting example.